Linux Networking Cli & Introduction to Yocto for Machine Routing

This documentation covers essential concepts and commands for the Linux Networking Command Line Interface, specifically tailored for machine routing and embedded systems context.

Listing Network Interfaces

To view all network interfaces on the system, including those that are currently "down" (inactive), you can use two main approaches: the legacy net-tools or the modern iproute2 suite.

Method A: Using ifconfig (legacy)

Note

ifconfig is part of the net-tools package. While widely used, it is technically deprecated in some modern distributions in favor of the ip command.

Command:

ifconfig -a

Output Explanation: Below is a breakdown of the output fields using the provided example:

enp52s0: flags=4099<UP,BROADCAST,MULTICAST>  mtu 1500
        ether e8:80:88:29:6a:93  txqueuelen 1000  (Ethernet)
        RX packets 0  bytes 0 (0.0 B)
        RX errors 0  dropped 0  overruns 0  frame 0
        TX packets 0  bytes 0 (0.0 B)
        TX errors 0  dropped 0 overruns 0  carrier 0  collisions 0

• Interface Name (enp52s0): Represents the physical Ethernet port.

• Flags <UP...>: The interface is enabled logically, but RUNNING is missing, indicating no cable is plugged in.

• ether: The MAC Address (Physical address) of the card.

• RX/TX: Receive/Transmit statistics. Here they are 0, confirming no traffic.

lo: flags=73<UP,LOOPBACK,RUNNING>  mtu 65536
        inet 127.0.0.1  netmask 255.0.0.0
        inet6 ::1  prefixlen 128  scopeid 0x10<host>
        loop  txqueuelen 1000  (Local Loopback)
        RX packets 152  bytes 14709 (14.7 KB)
        ...

• lo (Loopback): The internal virtual interface used for localhost communication.

• inet 127.0.0.1: The standard localhost IPv4 address.

• MTU 65536: Loopback supports huge packet sizes because traffic stays in memory.

wlp0s20f3: flags=4163<UP,BROADCAST,RUNNING,MULTICAST>  mtu 1500
        inet 10.165.26.87  netmask 255.255.255.0  broadcast 10.165.26.255
        inet6 fe80::7cf3:464:1386:165c  prefixlen 64  scopeid 0x20<link>
        ether 30:89:4a:c2:74:1e  txqueuelen 1000  (Ethernet)
        RX packets 95989  bytes 101998303 (101.9 MB)
        ...

• wlp... (Wireless): The Wi-Fi interface.

• flags <...RUNNING...>: Indicates the interface is active and connected to a network.

• inet 10.165.26.87: The IPv4 address assigned to this machine.

• RX/TX packets: Shows actual data traffic (101.9 MB received, 11.3 MB sent).

Method B: Using ip Link (modern Standard)

Note

This is the preferred method in modern Linux systems (Yocto/Embedded Linux often relies heavily on iproute2).

Command:

ip link show

Output Explanation:

1: lo: <LOOPBACK,UP,LOWER_UP> mtu 65536 qdisc noqueue state UNKNOWN mode DEFAULT group default qlen 1000
    link/loopback 00:00:00:00:00:00 brd 00:00:00:00:00:00

• state UNKNOWN: Normal for loopback interfaces (it doesn't need a physical carrier).

• qdisc noqueue: No queuing discipline needed (instant traffic).

2: enp52s0: <NO-CARRIER,BROADCAST,MULTICAST,UP> mtu 1500 qdisc fq_codel state DOWN mode DEFAULT group default qlen 1000
    link/ether e8:80:88:29:6a:93 brd ff:ff:ff:ff:ff:ff

• NO-CARRIER: Confirms physical disconnection (cable unplugged).

• state DOWN: The interface is administratively UP (enabled in OS), but physically DOWN.

• qdisc fq_codel: Fair Queuing Controlled Delay (modern algorithm to prevent buffer bloat).

3: wlp0s20f3: <BROADCAST,MULTICAST,UP,LOWER_UP> mtu 1500 qdisc noqueue state UP mode DORMANT group default qlen 1000
    link/ether 30:89:4a:c2:74:1e brd ff:ff:ff:ff:ff:ff

• state UP: The interface is fully operational and connected.

• LOWER_UP: Physical layer (L1) is active.

Summary Comparison: ifconfig vs ip Link Show

Interface Configuration & Management

This section covers how to manipulate the state of interfaces and assign addresses.

2.1 Enabling/disabling Interfaces

Purpose: Activate (bring up) or deactivate (take down) a specific network interface to allow or stop traffic.

Using ip (Modern):

# Bring interface UP
sudo ip link set dev <interface> up

# Bring interface DOWN
sudo ip link set dev <interface> down

• set: The action to modify the device.

• dev <interface>: Target device (e.g., eth0, wlan0).

• up/down: The desired state.

Using ifconfig (Legacy):

sudo ifconfig <interface> up
sudo ifconfig <interface> down

2.2 Configuring Ip Addresses

Purpose: Assign static IP addresses to network interfaces. This is common in server/embedded environments where DHCP is not used.

Using ip (Modern):

sudo ip addr add 192.168.1.100/24 dev <interface>
sudo ip link set <interface> up

• addr add: Command to append an address.

• 192.168.1.100/24: CIDR Notation. This is crucial.

  • 192.168.1.100 is the IP.

  • /24 represents the Subnet Mask (255.255.255.0).

• Note: If you make a mistake, use del instead of add to remove it.

Using ifconfig (Legacy):

sudo ifconfig <interface> 192.168.1.100 netmask 255.255.255.0 up

• netmask: explicitly defines the mask (older style compared to CIDR).

• up: Can be combined in one line to activate the interface immediately.

Important

These changes are temporary. They will be lost after a reboot. For persistent configuration in Yocto/Linux, you would edit files like /etc/network/interfaces or use systemd-networkd / NetworkManager.

Hostname and Dns Configuration

Setting the identity of the machine and how it finds other machines.

3.1 Managing Hostnames

Purpose: Set or change the system’s name.

• Temporary (Current Session only):

  sudo hostname <newhostname>

• Persistent (Permanent across reboots):

  sudo hostnamectl set-hostname <newhostname>

  • hostnamectl: The systemd utility to query and change the system hostname and related settings.

3.2 Editing the Hosts File

File Location: /etc/hosts

Purpose: Local DNS resolution. The system checks this file before asking a DNS server. It maps hostnames directly to IP addresses.

Example Entry:

127.0.0.1      localhost
192.168.1.10   myserver.local  myserver

• If you ping myserver, the system immediately knows it is 192.168.1.10 without needing network lookups.

3.3 Dns Setup

File Location: /etc/resolv.conf

Purpose: Defines the Nameservers (DNS servers) the system uses to resolve domain names (like google.com) to IPs.

Example Content:

nameserver 8.8.8.8
nameserver 1.1.1.1

Warning

In modern systems (Ubuntu, Yocto with systemd), /etc/resolv.conf is often a symlink managed by systemd-resolved or NetworkManager. Editing it manually might work temporarily, but changes could be overwritten by the OS.

Testing Connectivity

4.1 Ping Command

Purpose: Test connectivity by sending ICMP echo requests to a target.

Command:

ping <ip-address or hostname>

Explanation & Options:

• Sends packets and reports round-trip time (RTT).

• Press Ctrl + C to stop the command.

• Options like -c <count> can limit the number of pings (e.g., ping -c 4 google.com).

4.2 Traceroute

Purpose: Determine the route packets take to reach a network host.

Command:

traceroute <ip-address or hostname>

Explanation & Options:

• Displays each hop along the path.

• Options like -m <max hops> can set the maximum number of hops (e.g., traceroute -m 15 google.com).

4.3 Mtr

Purpose: Combine the functionalities of ping and traceroute for real-time network path analysis.

Command:

mtr <ip-address or hostname>

Explanation: Provides a continuously updated view of the route and latency at each hop. Options allow changing the reporting interval and packet size.

Analyzing Network Details

5.1 Netstat

Purpose: Display network connections, routing tables, interface statistics, and more.

Command:

netstat -tulpn

Explanation & Options:

• -t: Lists TCP connections.

• -u: Lists UDP connections.

• -l: Shows only listening sockets.

• -p: Displays the process using the socket.

• -n: Uses numeric addresses instead of resolving hostnames.

5.2 Ss Command

Purpose: Modern alternative to netstat for displaying socket statistics.

Command:

ss -tulpn

Explanation: Uses similar options to netstat with improved performance and additional filtering capabilities.

5.3 Ip Route

Purpose: View and manage routing tables.

Command:

ip route show

Explanation & Options:

• ip route: Manages routes in the kernel.

• show: Lists all current routes.

• Adding a route:

  sudo ip route add <destination> via <gateway> dev <interface>

  This command explains how packets to <destination> are routed through a specified <gateway> on the given <interface>.

Network Diagnostics and Troubleshooting

6.1 Tcpdump

Purpose: Capture and analyze network packets in real time.

Command:

sudo tcpdump -i <interface>

Explanation & Options:

• -i <interface>: Specifies which interface to capture packets from.

• -w <file>: Write the output to a file (e.g., -w capture.pcap).

• Filter expressions can narrow the capture (e.g., tcpdump -i <interface> port 80).

6.2 Nmap

Purpose: Network exploration and security auditing.

Command:

nmap <target-ip or hostname>

Explanation & Options:

• -F: Quick scan of the most common ports.

• -p <port-range>: Specify ports to scan (e.g., -p 1-65535).

• -O: Attempt to detect the operating system of the target (requires sudo for some scans).

6.3 Ethtool

Purpose: Query and control network device settings.

Command:

sudo ethtool <interface>

Explanation & Options:

• Displays detailed information such as speed, duplex, and auto-negotiation settings.

• To change settings:

  sudo ethtool -s <interface> speed 100 duplex full autoneg off

  This command sets the interface to 100Mb/s full duplex with auto-negotiation disabled.

6.4 Wireless (iw)

Purpose: Manage and troubleshoot wireless interfaces.

Commands:

• List wireless devices:

  iw dev
  

• Display connection status:

  iw <interface> link

• Scan for networks:

  iw <interface> scan
  

Explanation: The iw suite provides detailed control over wireless settings, replacing the older iwconfig tool.

Accessing Network Services

7.1 Ssh

Purpose: Securely access remote machines.

Command:

ssh username@server-ip

Explanation & Options:

• Establishes an encrypted connection to a remote host.

• Options can include specifying a port (-p <port>), using key authentication (-i <keyfile>), and more.

7.2 Ftp and Sftp

Purpose: Transfer files between systems.

• FTP Command:

  ftp <server-ip>

  Connects to an FTP server. Commands like put and get manage file transfers.

• SFTP Command:

  sftp username@server-ip

  Secure version running over SSH. It offers similar file operations as FTP but with encryption.

7.3 Telnet

Purpose: Test connectivity to a port (not recommended for secure operations).

Command:

telnet <server-ip> <port>

Explanation: Connects to the specified port on the remote server, useful for basic connectivity tests.

7.4 Web Tools: Curl and Wget

Purpose: Retrieve data from web servers or perform HTTP/S operations.

• curl:

  curl -I [https://www.example.com](https://www.example.com)

  • -I: Fetches only the HTTP headers.

  • Can also be used for POST, PUT, and other HTTP methods.

• wget:

  wget [https://www.example.com/file.tar.gz](https://www.example.com/file.tar.gz)

  • Downloads files from the specified URL. Supports options for recursion, limiting download speed, and more.

Firewall and Security Tools

8.1 Iptables/nftables

Purpose: Manage packet filtering and NAT rules.

• iptables Example:

  sudo iptables -L -n -v

  • -L: List all rules.

  • -n: Display numerical addresses.

  • -v: Verbose output.

• nftables Example:

  sudo nft list ruleset

  Displays all rules currently active under nftables, the modern replacement for iptables.

8.2 Ufw and Firewalld

Purpose: Simplify firewall management.

• ufw (Uncomplicated Firewall):

  sudo ufw enable
  sudo ufw allow 22/tcp
  sudo ufw status verbose

  Provides a simplified interface to configure iptables rules, ideal for users seeking ease of use.

• firewalld:

  sudo firewall-cmd --permanent --add-port=22/tcp
  sudo firewall-cmd --reload
  sudo firewall-cmd --list-all

  Used on many Red Hat-based systems to manage dynamic firewall rules with zones.

Key Configuration Files

Understanding where configuration changes persist is vital:

• /etc/hostname – Contains the system’s hostname.

• /etc/hosts – Maps hostnames to IP addresses.

• /etc/resolv.conf – Specifies DNS server addresses.

• /etc/network/interfaces or /etc/sysconfig/network-scripts/ifcfg-* – Contains interface configuration for various distros.

• /etc/ssh/sshd_config – SSH daemon configuration.

• /etc/services – Lists service names and their associated ports.

Mapping Commands to Network Layers (osi)

Understanding where each command operates helps in diagnostics.

Logical Troubleshooting Workflow

If the network is down, follow this bottom-up approach:

1. Physical Check (L1): Is the cable plugged in? Is the interface UP?

   • Command: ip link show (Look for LOWER_UP)

   • Fix: ip link set dev eth0 up

2. Link/Local Check (L2): Do you have a MAC address? Can you see neighbors?

   • Command: ip neigh (Are neighbors STALE or REACHABLE?)

3. IP Configuration (L3): Do you have an IP address?

   • Command: ip addr show

   • Fix: Check DHCP or assign static IP.

4. Gateway/Routing (L3): Can you reach the router?

   • Command: ip route show (Check for default via ...)

   • Test: ping <gateway_ip>

5. External Connectivity (L3): Can you reach the internet?

   • Test: ping 8.8.8.8 (Google DNS)

6. DNS Resolution (L7): Can you resolve names?

   • Test: ping google.com

   • Fix: Check /etc/resolv.conf

Yocto & Embedded Specifics

In Embedded Linux (Yocto), networking is slightly different than Desktop Linux.

1. Persistence: The ip commands above are temporary. They reset on reboot.

2. Network Managers: Yocto images typically use one of the following to manage network permanently:

   • systemd-networkd: (Modern, common) Configured in /etc/systemd/network/*.network.

   • ConnMan: (Common in mobile/IoT) specialized for embedded.

   • ifupdown: (Legacy) uses /etc/network/interfaces.

Example: Persistent Static IP in systemd-networkd Create a file /etc/systemd/network/20-wired.network:

[Match]
Name=eth0

[Network]
Address=192.168.1.10/24
Gateway=192.168.1.1
DNS=8.8.8.8

Additional Tools and Best Practices

13.1 Speedtest-cli

Purpose: Measure internet bandwidth using speedtest.net.

Command:

sudo apt install speedtest-cli  # For Debian/Ubuntu
speedtest-cli

Explanation: Run tests to determine your current download and upload speeds.

13.2 Additional Tips

• Automation: Consider automating common tasks with shell scripts or configuration management tools like Ansible.

• Documentation: Always consult the man pages (e.g., man ip, man tcpdump) for in-depth command options.

• Testing: Experiment in a lab environment before deploying changes to production networks.

Conclusion

This guide has expanded on basic Linux networking by providing detailed explanations for each command-line tool and its options. By understanding the OSI layers and following a logical troubleshooting workflow, you can effectively manage and debug any embedded Linux network.